In recent years, charged particle beam apparatuses such as a scanning electron microscope (SEM), a transmission electron microscope (TEM), a scanning and transmission electron microscope (STEM), a focused ion beam processing observation instrument (FIB) and the like have been used, in order to observe microscopic regions. In these apparatuses, a charged particle beam such as an electron beam or an ion beam is irradiated onto a sample. In general, these charged particle beam apparatuses are adapted to evacuate the air from a space in which a sample is placed, and perform imaging. There are plenty of needs to observe a sample using an electron microscope even under low-vacuum and/or under atmospheric pressure. Recently, SEMs that can observe a sample which is a target of observation under atmospheric pressure have been developed (see Patent Literature 1 and Patent Literature 2). These SEMs are in principle configured to provide a diaphragm through which an electron beam can pass, between an electron optical system and a sample, to thereby separate a space in vacuum state from a space in atmospheric pressure state. Any of the SEMs are common in point of providing the diaphragm between the sample and the electron optical system.
Patent Literature 1 discloses an atmospheric pressure SEM in which an electron source is arranged in the lower side of an electron optical lens barrel and an objective lens is arranged in the upper side thereof, and a diaphragm through which an electron beam can pass is provided via an O-ring on the electron-beam exit aperture at the end of the electron optical lens barrel. The sample is directly placed on the diaphragm and a primary electron beam is irradiated from the lower surface side of the sample to perform. SEM observation. Patent Literature 2 discloses arranging an electron source in the upper side of an electron optical lens barrel and an objective lens in the lower side thereof, and providing a diaphragm at a position away from a sample. An electron beam is irradiated from the upper surface side of the diaphragm to perform SEM observation. Note that the diaphragm is not only provided at the position away from the sample, but also provided near the electron source.